U.S. Pat. No. 4,004,462 to Robert C. Dobkin is assigned to the assignee of the present invention and is titled TEMPERATURE TRANSDUCER. This patent describes an integrated circuit in which a voltage is developed that is linear and directly proportional to absolute temperature (PTAT) and which extrapolates to zero at absolute zero. The characteristic is developed as a fundamental property of the differential in base to emitter voltage in a pair of transistors operated at different current densities. The nature of the device means that only one calibration point is needed to make it accurate over its entire range.
Commercially the LM135/LM235/LM335 Precision Temperature Sensor Family is available from National Semiconductor Corporation. These devices when biased from a nominal source of current develop a 10 mv/.degree.K voltage response, operate over the range of -55.degree. C. to +150.degree. C., and when calibrated at 25.degree. C. have less than 1.degree. C. error over a 100.degree. C. range. To obtain a Fahrenheit or Celsius scale reading the output of a sensor is combined with the output of a precision temperature-stable voltage that is designed to be equal to the temperature sensor voltage at the temperature scale's zero point. This is an undesirable approach because it requires a sensor along with a number of other stable, low-drift external components.
It is well recognized that a single IC chip could be provided with the circuits necessary to develop both a temperature-related voltage and a temperature-stable precision reference voltage. However, this would require a very complex IC design.
In the IEEE Journal of Solid-State Circuits for June, 1980, a paper titled "An IC Temperature Transducer with an Intrinsic Reference" by Gerard C. M. Meijer appears on pages 370-373. The approach is to develop a PTAT voltage and to subtract therefrom a voltage having a negative temperature coefficient, such as the base-to-emitter voltage of a conducting transistor. If the two voltages are made equal at zero Celsius, the result is a voltage that is proportional to the Celsius temperature scale. An amplifier that responds to the difference voltage can then be used to create a Celsius thermometer having a scale factor that is determined by amplifier gain.